Fuel box in a boiling water nuclear reactor

ABSTRACT

A method for manufacturing a sheet metal for use in a boiling water nuclear reactor and such a sheet metal. The method includes providing a material of a zirconium alloy that includes zirconium, and whose main alloying materials include niobium. The material is annealed so that essentially all niobium containing secondary phase particles are transformed to β-niobium particles.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. provisional patent application60/585,522 filed 6 Jul. 2004 and is the national phase under 35 U.S.C.§371 of PCT/SE2005/001000 filed 22 Jun. 2005.

TECHNICAL FIELD

The present invention relates to sheet metals in water boiling nuclearreactors and to a method for manufacturing of such sheet metals and tofuel boxes comprising such sheet metals.

DESCRIPTION OF THE PRIOR ART

In boiling water nuclear reactors the nuclear fuel is arranged in fuelpellets, which are arranged in fuel rods that in turn are arranged in afuel box. A fuel assembly comprises the fuel box, the therein arrangedfuel rods, spreader elements, and various other elements that are knownto persons skilled in the art. The fuel boxes are arranged as elongatedpipes with openings in the ends. The fuel boxes are manufactured fromsheet metals that are bent and welded together, which sheet metalsusually are comprised of a zirconium alloy. When zirconium alloys areexposed to neutron irradiation they grow. During operation of a boilingwater nuclear reactor the fuel boxes are exposed to hot water andneutron irradiation, which will lead to growth and corrosion of the fuelboxes. The magnitude of this neutron induced growth is different fordifferent alloys. When the material in the fuel box grows it may lead tothe bending of the fuel box. The useful life for a fuel box in a boilingwater nuclear reactor is dependent on the resistance against corrosionand the resistance against bending.

In the U.S. Pat. No. 5,805,656 a fuel box and a method for manufacturingsuch a fuel box are described. The problem that is intended to be solvedis to provide a fuel box with better resistance against neutron inducedgrowth and corrosion compared with prior known fuel boxes. This issolved in said U.S. patent by binding layers of different alloys in anouter layer and an inner layer in a sheet metal. The inner layer is ofan alloy that has a higher resistance against irradiation growth and theouter layer has a higher resistance against corrosion.

Even if a fuel box according to the U.S. patent provides favorableresistance against neutron induced growth and favorable corrosionresistance properties it is, however, complicated to manufacture sheetmetals with a plurality of layers. Thus, there is a need for analternative to known fuel boxes, which only comprises one layer andwhich has at least as favorable resistance against neutron inducedgrowth and corrosion as known sheet metals.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a homogeneous sheetmetal for a boiling water nuclear reactor and a method for manufacturingsuch a sheet metal, wherein the sheet metal, when it is exposed toneutron irradiation grows to a small extent compared with known sheetmetals for boiling water nuclear reactors.

A further object of the present invention is to provide a fuel box for aboiling water nuclear reactor and a method for manufacturing such a fuelbox, which fuel box has favorable resistance against corrosion andagainst neutron induced growth and which fuel box is manufactured of ahomogeneous material.

These objects are fulfilled with a sheet metal, a fuel box, a method formanufacturing of a sheet metal and a method for manufacturing a fuelbox.

A basic idea with the present invention is to provide a sheet metalwhich consists of a zirconium alloy, which comprises niobium containingsecondary phase particles that essentially only consists of μ-niobiumparticles.

With secondary phase particles is in this application meant particles inthe alloy which have another composition than the main part of thealloy. β-zirconium particles are particles in the zirconium alloy thatcontain zirconium and niobium, wherein the major part is zirconium.β-niobium particles are particles in the zirconium alloy that for themost part consist of niobium. The β-niobium particles comprise more than90 percent by weight niobium and preferably more than 99 percent byweight niobium.

When niobium is present in a zirconium alloy, niobium containingsecondary phase particles are formed, which are particles in thezirconium alloy that contain niobium or a mixture of niobium andzirconium. If niobium is present in the zirconium alloy in asufficiently high concentration secondary phase particles in a firstphase may be present as a mixture of β-zirconium particles and β-niobiumparticles, and in a second phase be present as only β-niobium particles.The first phase is stable at a higher temperature than the second phase.The phase boundary for secondary phase particles in the form ofβ-zirconium particles is the phase boundary between the first phase andthe second phase. If the temperature is kept at a certain level belowthe temperature for the phase boundary for secondary phase particles inthe form of β-zirconium particles the β-zirconium particles will betransformed into β-niobium particles.

In the description the term material is used for the object that isgoing through treatment steps until the material is ready-treated to asheet metal.

According to a first aspect of the present invention a method isprovided for manufacturing of a sheet metal for use in a boiling waternuclear reactor. The method comprises the step of providing a materialof a zirconium alloy, which mainly consists of zirconium, wherein themain alloying materials of the alloy comprises niobium, wherein thealloy comprises niobium containing secondary phase particles and whereinno alloying material is present in a content exceeding 1.6 percent byweight. The method further comprises the steps of subjecting thematerial to at least one hot-rolling, subjecting the material to atleast a first β-quenching, and to subject the hot-rolled material to atleast one cold-rolling. The method is characterized in that it furthercomprises the step of, after said at least one cold-rolling and aftersaid first β-quenching, transformation annealing the cold-rolledmaterial, at a temperature under the phase boundary for secondary phaseparticles in the form of β-zirconium particles, for so long time thatessentially all niobium containing secondary phase particles aretransformed into β-niobium particles, which are particles in thezirconium alloy with a niobium content exceeding 90 percent by weight.

With a method according to the invention a zirconium alloy is provided,which grows only to a small extent when being exposed to neutronirradiation, and which has favorable resistance against corrosion.Naturally, other steps may be included in a method according to theinvention. Furthermore, the different steps in the method according tothe invention may be performed in a different order.

β-quenching is well known to persons skilled in the art and implies thatthe zirconium alloy is heated to a high temperature, so that a crystalstructure of the type bcc (body center cubic) is obtained in thezirconium alloy, and that the zirconium alloy is then rapidly cooled sothat a crystal structure of the type hcp (hexagonal closed packed) isobtained. Through this method the zirconium alloy gets a randomizedstructure.

The first β-quenching may be performed before the hot-rolling. If thisis the case the structure in the material may be effected to some extentin the following hot-rolling and in further other following steps.Alternatively, the first β-quenching may be performed between two ofsaid at least one hot-rolling and said at least one cold-rolling orbetween hot-rollings.

To obtain a randomized structure of the alloy, a second β-quenching hasto be performed after the last cold-rolling.

In case the method comprises a second β-quenching at a late stage on analmost finished product it is advantageous that the method comprises acold deformation between the second β-quenching and the transformationannealing, wherein the material is stretched so that the remainingdeformation is 1%-7% of the original size before the stretching. With acold deformation before the transformation annealing the desired resultfor the composition of the secondary phase particles is achieved morerapidly.

The temperature during the transformation annealing effects the rate atwhich β-zirconium particles are transformed into β-niobium particles.The rate is dependent partly on the rate of diffusion at which niobiumdiffuses in zirconium and partly on the rate of nucleation at whichniobium particles are formed in zirconium. The rate of diffusionincreases with temperature while the rate of nucleation decreases withtemperature. The transformation annealing is performed at 450° C.-600°C., advantageously at 500° C.-600° C., and preferably at 520° C.-580° C.in order for the transformation to be rapid.

The time period during which the transformation annealing has to proceeddepends on the temperature. If the temperature is kept at 500° C.-600°C., the transformation annealing is preferably performed during 6-10hours and at least during more than or equal to 3 hours. At lowertemperatures the transformation annealing has to proceed for longertime.

To achieve good properties regarding corrosion and neutron inducedgrowth, the niobium content is preferably 0.5-1.6 percent by weight, theiron content is preferably 0.3-0.6 percent by weight and the tin contentis preferably 0.5-0.85 percent by weight. The group of alloys where allthree percentages are fulfilled is especially advantageous for achievingfavorable properties regarding corrosion and neutron induced growth.There might also be other materials present in the alloy, the content ofwhich, however, are below 0.05 percent by weight.

Another group of alloys with especially favorable properties is namedZirlo, in which group of alloys the tin content is 0.7-1.1 percent byweight, the iron content is 0.09-0.15 percent by weight and the niobiumcontent is 0.8-1.2 percent by weight. There might also be othermaterials in the alloy, the content of which, however, are below 0.05percent by weight.

As an alternative to the groups of alloys described above it is possibleto have an alloy that essentially only comprises niobium as an alloyingmaterial, wherein the niobium content is 0.5-1.6 percent by weight andpreferably 0.9-1.1 percent by weight. There might also be othermaterials in the alloy. The content of these other materials are,however, below 0.05 percent by weight.

Advantageously, the temperature after the transformation annealing doesnot exceed the temperature for the phase boundary for secondary phaseparticles in the form of β-zirconium particles. In case the temperatureafter the transformation annealing exceeds the temperature for the phaseboundary for secondary phase particles in the form of β-zirconiumparticles, it does that for at most so long time that essentially allniobium containing secondary phase particles are maintained as β-niobiumparticles. In order to achieve this the temperature, after thetransformation annealing, exceeds the temperature for the phase boundaryfor secondary phase particles in the form of β-zirconium particles,suitably for no more than 10 minutes, preferably for no more than 5minutes and advantageously not at all. The time depends on how much thetemperature is allowed to exceed the temperature for the phase boundaryfor β-zirconium particles.

According to a second aspect of the present invention a method isprovided for manufacturing a fuel box for a boiling water nuclearreactor, wherein a sheet metal is manufactured, and wherein the sheetmetal is arranged as at least one of the walls of the fuel box.

According to a third aspect of the present invention a sheet metal isprovided for use in a boiling water nuclear reactor, which sheet metalis comprised of a zirconium alloy which mainly consists of zirconium,wherein the main alloying materials of the alloy comprises niobium,wherein no alloying material is present in a content in excess of 1.6percent by weight and wherein the alloy comprises niobium containingsecondary phase particles. The sheet metal is characterized in that theniobium containing secondary phase particles essentially only consistsof β-niobium particles, which are particles in the zirconium alloy witha niobium content that exceeds 90 percent by weight and preferablyexceeds 99 percent by weight.

The sheet metal has the advantages that have been described above inrelation to the method according to the present invention. The sheetmetal may of course be manufactured with a method as described above.

According to a fourth aspect of the present invention a fuel box isprovided, which comprises a sheet metal according to the inventionarranged as at least one of the walls of the fuel box.

The features that have been described in relation to the method abovemay, where it is applicable, also be applied to a sheet metal and a fuelbox according to the invention.

It goes without saying that the different features that have beendescribed above, may be combined in the same embodiment where it isapplicable. In the following different embodiments of the invention willbe described with reference to the accompanying drawings.

SHORT DESCRIPTION OF THE DRAWING

FIG. 1 shows a fuel assembly including a fuel box according to anembodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a fuel assembly 1 according to the prior art, which isarranged for a boiling water nuclear reactor. The fuel assembly 1comprises a fuel box 2 according to an embodiment of the presentinvention. The fuel assembly also comprises fuel rods 3 in which thenuclear fuel is arranged in fuel pellets. The fuel box 2 has a lengthaxis 4 which is parallel to the length axis of the fuel rods 3. The fuelbox 2 is usually manufactured of two sheet metals 5 which are bent andwelded together along the direction of the length axis 4 of the fuel box2.

Below two examples are given on manufacturing methods according to theinvention for a sheet metal 5 for a fuel box 2.

Common for the methods is that a transformation annealing is performedat a late stage in order to transform the secondary phase particles inthe form of β-zirconium particles into secondary phase particles in theform of β-niobium particles. The annealing is performed at a temperaturethat is below the temperature for the phase boundary for β-zirconiumparticles, which is at approximately 610° C. The driving force for thetransformation of secondary phase particles from β-zirconium particlesto β-niobium particles is partly limited by the diffusion rate forniobium in zirconium and partly limited by the nucleation rate, which isthe rate at which secondary phase particles are formed in the alloy. Thediffusion increases with an increasing temperature while the nucleationrate decreases with an increasing temperature. This implies that thereis an optimal temperature for a high transformation rate.

For the alloys that are contemplated in this application the optimaltemperature for a rapid transformation is approximately 550° C. However,it is possible to achieve the desired result as long as the temperatureis below the temperature for the phase boundary for β-zirconiumparticles. A preferred interval is 500-600° C. and an even morepreferred interval is 540-580° C.

The alloys that primarily are interesting for the invention are the onesthat have a niobium content of 0.5-1.6 percent by weight.

A first group of alloys are the ones that have the niobium contentmentioned above, an iron content of 0.3-0.6 percent by weight and a tincontent of 0.5-0.85 percent by weight. There may also be other materialsin the alloy. The content of these other materials is, however, below0.05 percent by weight.

A second group of alloys is Zirlo that has 0.7-1.1 percent by weighttin, 0.09-0.15 percent by weight iron, 0.8-1.2 percent by weightniobium. There might also be other materials in the alloy, the contentof which, however, is below 0.05 percent by weight.

A third group of alloys comprises as a main alloying material onlyniobium, wherein the niobium content is 0.9-1.1 percent by weight. Theremight also be other materials in the alloy, the content of which,however, is below 0.05 percent by weight.

All these groups of alloys provides favorable resistance againstcorrosion and little neutron induced growth.

Example 1

When manufacturing the sheet metal 5 in the fuel box 2 according to afirst example, firstly an electrode of a zirconium alloy ismanufactured, which comprises approximately 1 percent by weight ofniobium, 0.4 percent by weight of iron and 0.6 percent by weight of tinbased on the weight of the electrode, by pressing together zirconiumbriquettes together with alloying materials. Thereafter, the electrodeis vacuum melted to a casting which thereafter is vacuum melted at leastonce, whereupon the casting is forged to a material which is 100-125 mmthick, which in turn is worked and surface conditioned. Thereafter thematerial is subject to β-quenching, which implies that the material isheated to a temperature of 1000° C.-1100° C. and thereafter is cooled.The material is cooled at a rate of at least 10° C. per second to atemperature below 500° C. After the β-quenching the material is surfaceconditioned and is then hot-rolled in several steps. The number of stepsand the thicknesses after each hot-rolling depends on the finalthickness that is desired on the sheet metal 5.

The material is subject to a number of cold-rollings. By subjecting thehot-rolled material to annealing before the first cold-rolling afavorable grain structure is obtained in the material. Between each oneof the cold-rollings the material is annealed in order to restore thegrain structure before the next cold-rolling, according to standardmanufacturing procedures. Annealing is performed at a temperature belowthe temperature at β-quenching, i.e., below 900° C. and preferably belowapproximately 600° C., for example at approximately 560° C.

After the cold-rollings a transformation annealing is performed byheating the material to a temperature of 545° C. for six hours. Duringthe transformation annealing secondary phase particles in the form ofβ-zirconium particles are transformed into secondary phase particles inthe form of β-niobium particles, which consist of particles with aniobium content that exceeds 99 percent by weight.

After the transformation annealing the material is cold-rolled to afinished dimension and is finish annealed in order to restore the grainstructure. The finish annealing is performed at a temperature that isbelow the temperature for the phase boundary for β-zirconium particles.The finished sheet metal has thereby been manufactured. Finally, theedges of the sheet metal 5 are cut, which sheet metal is also surfaceconditioned.

Example 2

When manufacturing the sheet metal 5 in the fuel box 2 according to asecond example, an electrode of a zirconium alloy is manufactured, whichcomprises 0.97 percent by weight of tin, 0.01 percent by weight of iron,1.03 percent by weight of niobium, and 0.0081 percent by weight ofchromium, by pressing together zirconium briquettes together with thealloying materials. This alloy is also known under the name Zirlo.Thereafter, the electrode is vacuum melted to a casting which thereafteris vacuum re-melted at least once, whereupon the casting is forged to amaterial which is 100-125 mm thick, which in turn is worked and surfaceconditioned. Thereafter the material is subject to β-quenching, whichimplies that the material is heated to a temperature of 1000° C.-1100°C. and thereafter is cooled rapidly. The material is cooled at a rate ofat least 10° C. per second to a temperature below 500° C. Then thematerial is hot-rolled in several steps. The number of steps and thethicknesses after each hot-rolling depends on the final thickness thatis desired on the sheet metal 5.

By subjecting the hot-rolled material to annealing before the firstcold-rolling a favorable grain structure is obtained in the material.The material is subject to a number of cold rollings. Between each oneof the cold-rollings the material is annealed in order to restore thegrain structure before the next cold-rolling, according to standardmanufacturing procedures. Annealing is performed at a temperature belowthe temperature at β-quenching, i.e. below 900° C. and preferably belowapproximately 600° C., for example at approximately 560° C.

Thereafter the material is subject to a second β-quenching, whichimplies that the material is heated to a temperature of 1000° C.-1100°C. and is then cooled rapidly. The material is cooled at a rate of atleast 10° C. per second to a temperature below 500° C.

After the second β-quenching a cold deformation is performed, whereinthe material is stretched so that the remaining deformation is 3% of theoriginal size before stretching. Thereafter a transformation annealingis performed by heating the material to a temperature of 545° C. duringsix hours. During the transformation annealing secondary phase particlesof β-zirconium particles are transformed to secondary phase particles inthe form of β-niobium particles, which consist of particles with aniobium content that exceeds 99 percent by weight. The finished sheetmetal 5 has thus been manufactured. Finally, the edges of the sheetmetal 5 are cut, which sheet metal is also surface conditioned.

After manufacturing the sheet metal according to any one of the aboveexamples a fuel box 2 is manufactured by bending two sheet metals 5 andwelding them together to a fuel box 2. The way a fuel box 2 ismanufactured from sheet metals 5 is known from the art and will not bedescribed in detail here.

Naturally, the invention is not limited to the embodiments describedabove but may be modified in numerous ways without departing from thescope of the present invention, which is limited only by the appendedclaims.

It is possible to include also other alloying materials than the onesthat have been mentioned above in concentrations below theconcentrations of the alloying materials mentioned above.

Naturally, it is possible to exchange the alloying materials for eachother in de embodiments described above or to replace them with an alloycomprising only niobium as an alloying material.

1. A method for manufacturing of a sheet metal for use in a boilingwater nuclear reactor, the method comprising: providing a material of azirconium alloy, comprising zirconium, wherein the main alloyingmaterials of the alloy comprises niobium, wherein no alloying materialis present in a content exceeding 1.6 percent by weight and wherein thealloy comprises niobium containing secondary phase particles, subjectingthe material to at least one hot-rolling, subjecting the material to atleast a first β-quenching, subjecting the hot-rolled material to atleast one cold-rolling, and, after said at least one cold-rolling andafter said first β-quenching, transformation annealing the cold-rolledmaterial, at a temperature below the phase boundary for secondary phaseparticles in the form of β-zirconium particles, for so long time thatessentially all niobium containing secondary phase particles aretransformed into β-niobium particles, which are particles in thezirconium alloy with a niobium content exceeding 90 percent by weight,wherein the main alloying materials are niobium, iron and tin, whereinthe tin content is 0.7-1.1 percent by weight, the iron content is0.09-0.15 percent by weight, and the niobium content is 0.8-1.2 percentby weight.
 2. The method according to claim 1, wherein the firstβ-quenching is performed before the hot-rolling.
 3. The method accordingto claim 1, wherein the first β-quenching is performed between one ofsaid at least one hot-rolling and said at least one cold-rolling.
 4. Themethod according to claim 1, further comprising: arranging the sheetmetal as at least one of a fuel box.
 5. The method according to claim 1,further comprising: a second β-quenching, which is performed after saidat least one cold rolling and before the transformation annealing. 6.The method according to claim 5, further comprising: a cold deformationbetween the second β-quenching and the transformation annealing, whereinthe material during the cold deformation is stretched so that theremaining deformation is 1%-7% of the original size before thestretching.
 7. The method according to claim 1, wherein thetransformation annealing is performed at 450° C.-600° C.
 8. The methodaccording to claim 7, wherein the transformation annealing is performedat 500° C.-600° C.
 9. The method according to claim 7, wherein thetransformation annealing is performed at 540° C.-580° C.
 10. The methodaccording to claim 1, wherein the temperature, in case it after thetransformation annealing exceeds the temperature for the phase boundaryfor secondary phase particles in the form of β-zirconium particles, doesit for at most so long time that essentially all niobium containingsecondary phase particles are maintained as β-niobium particles.
 11. Themethod according to claim 10, wherein the temperature after thetransformation annealing exceeds the temperature for the phase boundaryfor secondary phase particles in the form of β-zirconium particles forno longer than 10 minutes.
 12. The method according to claim 11, whereinthe temperature after the transformation annealing exceeds thetemperature for the phase boundary for secondary phase particles in theform of 13-zirconium particles for no longer than 5 minutes.
 13. Themethod according to claim 11, wherein the temperature after thetransformation annealing does not exceed the temperature for the phaseboundary for secondary phase particles in the form of β-zirconiumparticles.
 14. A method for manufacturing of a sheet metal for use in aboiling water nuclear reactor, the method comprising: providing amaterial of a zirconium alloy, comprising zirconium, wherein the mainalloying materials of the alloy comprises niobium, wherein no alloyingmaterial is present in a content exceeding 1.6 percent by weight andwherein the alloy comprises niobium containing secondary phaseparticles, subjecting the material to at least one hot-rolling,subjecting the material to at least a first β-quenching, subjecting thehot-rolled material to at least one cold-rolling, and after said atleast one cold-rolling and after said first β-quenching, transformationannealing the cold-rolled material, at a temperature below the phaseboundary for secondary phase particles in the form of β-zirconiumparticles, for so long time that essentially all niobium containingsecondary phase particles are transformed into β-niobium particles,which are particles in the zirconium alloy with a niobium contentexceeding 90 percent by weight, wherein the main alloying materials areniobium, iron and tin, wherein the niobium content is 0.5-1.6 percent byweight, the iron content is 0.3-0.6 percent by weight, and the tincontent is 0.5-0.85 percent by weight.